Packaging Methods of Semiconductor X-Ray Detectors

ABSTRACT

Disclosed herein is a method for making an apparatus suitable for detecting X-ray, the method comprising: bonding a plurality of chips to a substrate; wherein the substrate comprises an X-ray absorption layer comprising a first plurality of electrical contacts; wherein each of the plurality of chips comprises an electronic layer comprising a second plurality of electrical contacts and an electronic system configured to process or interpret signals generated by X-ray photons incident on the X-ray absorption layer; aligning the first plurality of electrical contacts to the second plurality of electrical contacts; mounting the chips to the substrate such that the first plurality of electrical contacts are electrically connected to the second plurality of electrical contacts; wherein the second plurality of electrical contacts are configured to feed the signals to the electronic system.

TECHNICAL FIELD

The disclosure herein relates to X-ray detectors, particularly relatesto methods of packaging semiconductor X-ray detectors.

BACKGROUND

X-ray detectors may be devices used to measure the flux, spatialdistribution, spectrum or other properties of X-rays.

X-ray detectors may be used for many applications. One importantapplication is imaging. X-ray imaging is a radiography technique and canbe used to reveal the internal structure of a non-uniformly composed andopaque object such as the human body.

Early X-ray detectors for imaging include photographic plates andphotographic films. A photographic plate may be a glass plate with acoating of light-sensitive emulsion. Although photographic plates werereplaced by photographic films, they may still be used in specialsituations due to the superior quality they offer and their extremestability. A photographic film may be a plastic film (e.g., a strip orsheet) with a coating of light-sensitive emulsion.

In the 1980s, photostimulable phosphor plates (PSP plates) becameavailable. A PSP plate may contain a phosphor material with colorcenters in its lattice. When the PSP plate is exposed to X-ray,electrons excited by X-ray are trapped in the color centers until theyare stimulated by a laser beam scanning over the plate surface. As theplate is scanned by laser, trapped excited electrons give off light,which is collected by a photo multiplier tube. The collected light isconverted into a digital image. In contrast to photographic plates andphotographic films, PSP plates can be reused.

Another kind of X-ray detectors are X-ray image intensifiers. Componentsof an X-ray image intensifier are usually sealed in a vacuum. Incontrast to photographic plates, photographic films, and PSP plates,X-ray image intensifiers may produce real-time images, i.e., do notrequire post-exposure processing to produce images. X-ray first hits aninput phosphor (e.g., cesium iodide) and is converted to visible light.The visible light then hits a photocathode (e.g., a thin metal layercontaining cesium and antimony compounds) and causes emission ofelectrons. The number of emitted electrons is proportional to theintensity of the incident X-ray. The emitted electrons are projected,through electron optics, onto an output phosphor and cause the outputphosphor to produce a visible-light image.

Scintillators operate somewhat similarly to X-ray image intensifiers inthat scintillators (e.g., sodium iodide) absorb X-ray and emit visiblelight, which can then be detected by a suitable image sensor for visiblelight. In scintillators, the visible light spreads and scatters in alldirections and thus reduces spatial resolution. Reducing thescintillator thickness helps to improve the spatial resolution but alsoreduces absorption of X-ray. A scintillator thus has to strike acompromise between absorption efficiency and resolution.

Semiconductor X-ray detectors largely overcome this problem by directconversion of X-ray into electric signals. A semiconductor X-raydetector may include a semiconductor layer that absorbs X-ray inwavelengths of interest. When an X-ray photon is absorbed in thesemiconductor layer, multiple charge carriers (e.g., electrons andholes) are generated and swept under an electric field towardselectrical contacts on the semiconductor layer. Cumbersome heatmanagement required in currently available semiconductor X-ray detectors(e.g., Medipix) can make a detector with a large area and a large numberof pixels difficult or impossible to produce.

SUMMARY

Disclosed herein is a method for making an apparatus suitable fordetecting X-ray, the method comprising: bonding a plurality of chips toa substrate; wherein the substrate comprises an X-ray absorption layercomprising a first plurality of electrical contacts; wherein each of theplurality of chips comprises an electronic layer comprising a secondplurality of electrical contacts and an electronic system configured toprocess or interpret signals generated by X-ray photons incident on theX-ray absorption layer; aligning the first plurality of electricalcontacts to the second plurality of electrical contacts; mounting thechips to the substrate such that the first plurality of electricalcontacts are electrically connected to the second plurality ofelectrical contacts; wherein the second plurality of electrical contactsare configured to feed the signals to the electronic system.

According to an embodiment, the method further comprises attaching heplurality of chips to a support wafer.

According to an embodiment, the plurality of chips are attached to thesupport wafer with an adhesive.

According to an embodiment, the plurality of chips are attached to thesupport wafer after the plurality of chips are mounted to the substrate.

According to an embodiment, the plurality of chips are mounted to asecond substrate.

According to an embodiment, the method further comprises removing thesupport wafer.

According to an embodiment, removing the support wafer comprisesgrinding or etching the support wafer.

According to an embodiment, the method further comprises encapsulatingthe plurality of chips in a matrix.

According to an embodiment, the matrix comprises a polymer or glass.

According to an embodiment, the matrix fills gaps between the chips.

According to an embodiment, the method further comprises exposing asurface of each of the chips.

According to an embodiment, mounting the chips to the substratecomprises mounting the chips encapsulated in the matrix.

According to an embodiment, the electronic layer comprises viasextending to a surface opposite to the X-ray absorption layer.

According to an embodiment, the method further comprises aligning thevias to contact pads on an interposer substrate, and bonding the chipsto the interposer substrate such that the vias are electricallyconnected to the contact pads.

According to an embodiment, the interposer substrate comprisestransmission lines electrically connected to the contact pads andconfigured to route a signal on the contact pads to bonding pads on anedge of the interposer substrate.

According to an embodiment, the method further comprises counting theinterposer substrate to a printed circuit board or positioning theinterposer substrate side-by-side with a printed circuit board.

According to an embodiment, the electronic layer comprises a thirdplurality of electrical contacts configured to read output from theelectronic system or to provide power or a reference voltage to theelectronic system.

According to an embodiment, the X-ray absorption layer comprises afourth plurality of electrical contacts configured to connect with thethird electrical contacts when the chips are mounted to the substrate.

According to an embodiment, the X-ray absorption layer further comprisestransmission lines configured to route a signal at the fourth pluralityof electrical contacts to bonding pads on the X-ray absorption layer.

BRIEF DESCRIPTION OF FIGURES

FIG. 1A schematically shows a cross-sectional view of the detector,according to an embodiment.

FIG. 1B schematically shows a detailed cross-sectional view of thedetector, according to an embodiment.

FIG. 1C schematically shows an alternative detailed cross-sectional viewof the detector, according to an embodiment.

FIG. 2 schematically shows that the device may have an array of pixels,according to an embodiment.

FIG. 3 schematically shows a cross-sectional view of an electronicslayer in the detector, according to an embodiment.

FIG. 4A-FIG. 4C schematically show a process of packaging the detector100, according to an embodiment.

FIG. 5A-FIG. 5F schematically show a process of mounting a plurality ofchips onto a substrate, according to an embodiment.

FIG. 6A-FIG. 6E schematically show a process of mounting a plurality ofchips onto a substrate, according to an embodiment.

FIG. 7A-FIG. 7C schematically show a process f mounting a plurality ofchips onto a substrate, according to an embodiment.

FIG. 8A-FIG. 8E schematically show a process of mounting a plurality ofchips onto a substrate, according to an embodiment.

FIGS. 8F-8I schematically show routing of signal in the X-ray absorptionlayer and the electronic layer.

FIG. 9 schematically shows a system comprising the semiconductor X-raydetector described herein, suitable for medical imaging such as chestX-ray radiography, abdominal X-ray radiography, etc., according to anembodiment

FIG. 10 schematically shows a system comprising the semiconductor X-raydetector described herein suitable for dental X-ray radiography,according to an embodiment.

FIG. 11 schematically shows a cargo scanning or non-intrusive inspection(NII) system comprising the semiconductor X-ray detector describedherein, according to an embodiment.

FIG. 12 schematically shows another cargo scanning or non-intrusiveinspection (NII) system comprising the semiconductor X-ray detectordescribed herein, according to an embodiment.

FIG. 13 schematically shows a full-body scanner system comprising thesemiconductor X-ray detector described herein, according to anembodiment.

FIG. 14 schematically shows an X-ray computed tomography (X-ray CT)system comprising the semiconductor X-ray detector described herein,according to an embodiment.

FIG. 15 schematically shows an electron microscope comprising thesemiconductor X-ray detector described herein, according to anembodiment.

FIG. 16A and FIG. 16B each show a component diagram of an electronicsystem of the detector in FIG. 1A or FIG. 1B, according to anembodiment.

FIG. 17 schematically shows a temporal change of the electric currentflowing through an electrode (upper curve) of a diode or an electricalcontact of a resistor of an X-ray absorption layer exposed to X-ray, theelectric current caused by charge carriers generated by an X-ray photonincident on the X-ray absorption layer, and a corresponding temporalchange of the voltage of the electrode (lower curve), according to anembodiment.

FIG. 18 schematically shows a temporal change of the electric currentflowing through the electrode (upper curve) caused by noise (e.g., darkcurrent), and a corresponding temporal change of the voltage of theelectrode (lower curve), in the electronic system operating in the wayshown in FIG. 8, according to an embodiment.

FIG. 19 schematically shows a temporal change of the electric currentflowing through an electrode (upper curve) of the X-ray absorption layerexposed to X-ray, the electric current caused by charge carriersgenerated by an X-ray photon incident on the X-ray absorption layer, anda corresponding temporal change of the voltage of the electrode (lowercurve), when the electronic system operates to detect incident X-rayphotons at a higher rate, according to an embodiment.

FIG. 20 schematically shows a temporal change of the electric currentflowing through the electrode (upper curve) caused by noise (e.g., darkcurrent), and a corresponding temporal change of the voltage of theelectrode (lower curve), in the electronic system operating in the wayshown in FIG. 10, according to an embodiment.

FIG. 21 schematically shows a temporal change of the electric currentflowing through the electrode (upper curve) caused by charge carriersgenerated by a series of X-ray photons incident on the X-ray absorptionlayer, and a corresponding temporal change of the voltage of theelectrode, in the electronic system operating in the way shown in FIG.10 with RST expires before t_(e), according to an embodiment.

DETAILED DESCRIPTION

FIG. 1A schematically shows a cross-sectional view of the detector 100,according to an embodiment. The detector 100 may include an X-rayabsorption layer 110 and an electronics layer 120 (e.g., an ASIC) forprocessing or analyzing electrical signals incident X-ray generates inthe X-ray absorption layer 110. In an embodiment, the detector 100 doesnot comprise a scintillator. The X-ray absorption layer 110 may includea semiconductor material such as, silicon, germanium, GaAs, CdTe,CdZnTe, or a combination thereof. The semiconductor may have a high massattenuation coefficient for the X-ray energy of interest.

As shown in a detailed cross-sectional view of the detector 100 in FIG.1B, according to an embodiment, the X-ray absorption layer 110 mayinclude one or more diodes (e.g., p-i-n or p-n) formed by a first dopedregion 111, one or more discrete regions 114 of a second doped region113. The second doped region 113 may be separated from the first dopedregion 111 by an optional the intrinsic region 112. The discreteportions 114 are separated from one another by the first doped region111 or the intrinsic region 112. The first doped region 111 and thesecond doped region 113 have opposite types of doping (e.g., region 111is p-type and region 113 is n-type, or region 111 is n-type and region113 is p-type). In the example in FIG. 1B, each of the discrete regions114 of the second doped region 113 forms a diode with the first dopedregion 111 and the optional intrinsic region 112. Namely, in the examplein FIG. 1B, the X-ray absorption layer 110 has a plurality of diodeshaving the first doped region 111 as a shared electrode. The first dopedregion 111 may also have discrete portions.

When an X-ray photon hits the X-ray absorption layer 110 includingdiodes, the X-ray photon may be absorbed and generate one or more chargecarriers by a number of mechanisms. An X-ray photon may generate 10 to100000 charge carriers. The charge carriers may drift to the electrodesof one of the diodes under an electric field. The field may be anexternal electric field. The electrical contact 119B may includediscrete portions each of which is in electrical contact with thediscrete regions 114. In an embodiment, the charge carriers may drift indirections such that the charge carriers generated by a single X-rayphoton are not substantially shared by two different discrete regions114 (“not substantially shared” here means less than 2%, less than 0.5%,less than 0.1%, or less than 0.01% of these charge carriers flow to adifferent one of the discrete regions 114 than the rest of the chargecarriers). Charge carriers generated by an X-ray photon incident aroundthe footprint of one of these discrete regions 114 are not substantiallyshared with another of these discrete regions 114. A pixel 150associated with a discrete region 114 may be an area around the discreteregion 114 in which substantially all (more than 98%, more than 99.5%,more than 99.9%, or more than 99.99% of) charge carriers generated by anX-ray photon incident therein flow to the discrete region 114. Namely,less than 2%, less than 1%, less than 0.1%, or less than 0.01% of thesecharge carriers flow beyond the pixel.

As shown in an alternative detailed cross-sectional view of the detector100 in FIG. 1C, according to an embodiment, the X-ray absorption layer110 may include a resistor of a semiconductor material such as, silicon,germanium, GaAs, CdTe, CdZnTe, ora combination thereof, but does notinclude a diode. The semiconductor may have a high mass attenuationcoefficient for the X-ray energy of interest.

When an X-ray photon hits the X-ray absorption layer 110 including aresistor but not diodes, it may be absorbed and generate one or morecharge carriers by a number of mechanisms. An X-ray photon may generate10 to 100000 charge carriers. The charge carriers may drift to theelectrical contacts 119A and 119B under an electric field. The field maybe an external electric field. The electrical contact 119B includesdiscrete portions. In an embodiment, the charge carriers may drift indirections such that the charge carriers generated by a single X-rayphoton are not substantially shared by two different discrete portionsof the electrical contact 119B (“not substantially shared” here meansless than 2%, less than 0.5%, less than 0.1%, or less than 0.01% ofthese charge carriers flow to a different one of the discrete portionsthan the rest of the charge carriers). Charge carriers generated by anX-ray photon incident around the footprint of one of these discreteportions of the electrical contact 1193 are not substantially sharedwith another of these discrete portions of the electrical contact 119B.A pixel 150 associated with a discrete portion of the electrical contact1193 may be an area around the discrete portion in which substantiallyall (more than 98%, more than 99.5%, more than 99.9% or more than 99.99%of) charge carriers generated by an X-ray photon incident therein flowto the discrete portion of the electrical contact 119B. Namely, lessthan 2%, less than 0.5%, less than 0.1%, or less than 0.01% of thesecharge carriers flow beyond the pixel associated with the one discreteportion of the electrical contact 119B.

The electronics layer 120 may include an electronic system 121 suitablefor processing or interpreting signals generated by X-ray photonsincident on the X-ray absorption layer 110. The electronic system 121may include an analog circuitry such as a filter network, amplifiers,integrators, and comparators, or a digital circuitry such as amicroprocessors, and memory. The electronic system 121 may includecomponents shared by the pixels or components dedicated to a singlepixel. For example, the electronic system 121 may include an amplifierdedicated to each pixel and a microprocessor shared among all thepixels. The electronic system 121 may be electrically connected to thepixels by vias 131. Space among the vias may be filled with a fillermaterial 130, which may increase the mechanical stability of theconnection of the electronics layer 120 to the X-ray absorption layer110. Other bonding techniques are possible to connect the electronicsystem 121 to the pixels without using vias.

FIG. 2 schematically shows that the detector 100 may have an array ofpixels 150. The array may be a rectangular array, a honeycomb array, ahexagonal array or any other suitable array. Each pixel 150 may beconfigured to detect an X-ray photon incident thereon, measure theenergy of the X-ray photon, or both. For example, each pixel 150 may beconfigured to count numbers of X-ray photons incident thereon whoseenergy falls in a plurality of bins, within a period of time. All thepixels 150 may be configured to count the numbers of X-ray photonsincident thereon within a plurality of bins of energy within the sameperiod of time. Each pixel 150 may have its own analog-to-digitalconverter (ADC) configured to digitize an analog signal representing theenergy of an incident X-ray photon into a digital signal. The ADC mayhave a resolution of 10 bits or higher. Each pixel 150 may be configuredto measure its dark current, such as before or concurrently with eachX-ray photon incident thereon. Each pixel 150 may be configured todeduct the contribution of the dark current from the energy of the X-rayphoton incident thereon. The pixels 150 may be configured to operate inparallel. For example, when one pixel 150 measures an incident X-rayphoton, another pixel 150 may be waiting for an X-ray photon to arrive.The pixels 150 may be but do not have to be individually addressable.

FIG. 3 schematically shows the electronics layer 120 according to anembodiment. The electronic layer 120 comprises a substrate 122 having afirst surface 124 and a second surface 128. A “surface” as used hereinis not necessarily exposed, but can be buried wholly or partially. Theelectronic layer 120 comprises one or more electric contacts 125 on thefirst surface 124. The one or more electric contacts 125 may beconfigured to be electrically connected to one or more electricalcontacts 119B of the X-ray absorption layer 110. The electronics system121 may be in or on the substrate 122. The electronic layer 120comprises one or more vias 126 extending from the first surface 124 tothe second surface 128.

The substrate 122 may be a thinned substrate. For example, the substratemay have at thickness of 750 microns or less, 200 microns or less, 100microns or less, 50 microns or less, 20 microns or less, or 5 microns orless. The substrate 122 may be a silicon substrate or a substrate orother suitable semiconductor or insulator. The substrate 122 may beproduced by grinding a thicker substrate to a desired thickness.

The one or more electric contacts 125 may be a layer of metal or dopedsemiconductor. For example, the electric contacts 125 may be gold,copper, platinum, palladium, doped silicon, etc.

The vias 126 pass through the substrate 122 and electrically connectelectrical components (e.g., the electrical contacts 125 and theelectronic system 121) on the first surface 124 to electrical componentson the second surface 128. The vias 126 are sometimes referred to as“through-silicon vias” although they may be fabricated in substrates ofmaterials other than silicon. Multiple electronical components on thefirst surface 124 may share one via 126.

FIG. 3 further schematically shows bonding between the X-ray absorptionlayer 110 and the electronic layer 120 at the electrical contact 119Band the electrical contacts 125. The bonding may be by a suitabletechnique such as direct bonding or flip chip bonding.

Direct bonding is a wafer bonding process without any additionalintermediate layers (e.g., solder bumps). The bonding process is basedon chemical bonds between two surfaces. Direct bonding may be atelevated temperature but not necessarily so.

Flip chip bonding uses solder bumps 199 deposited onto contact pads(e.g., the electrical contact 119B of the X-ray absorption layer 110 orthe electrical contacts 125). Either the X-ray absorption layer 110 orthe electronic layer 120 is flipped over and the electrical contact 1193of the X-ray absorption layer 110 are aligned to the electrical contacts125. The solder bumps 199 may be melted to solder the electrical contact1193 and the electrical contacts 125 together. Any void space among thesolder bumps 199 may be filled with an insulating material.

FIG. 4A-FIG. 4C schematically show a process of packaging the detector100, according to an embodiment.

FIG. 4A schematically shows that multiple chips are obtained. Each ofthe chips includes the electronic layer 120 and the electrical contacts125. The chips may be obtained by dicing a wafer with multiple dies.

FIG. 4B schematically shows that the electrical contacts 125 of thechips are aligned to the electrical contacts 119B of the X-rayabsorption layer 110. In this view, the electrical contacts 125 are notvisible because they face the X-ray absorption layer 110 but theelectrical contacts 119B are visible.

FIG. 4C schematically shows that the chips are bonded to the X-rayabsorption layer 110 using a suitable bonding method. The electricalcontacts 119B of the X-ray absorption layer 110 are now electricallyconnected to the electrical contacts 125 of the electronic layer 120.

FIG. 5A-FIG. 5F schematically show a process of mounting a plurality ofchips onto a substrate, according to an embodiment. This process may beused in mounting the chips depicted in FIG. 4C.

FIG. 5A schematically shows that chips 810 (e.g., the chip including theelectronic layer 120 as shown in FIG. 4C) may be obtained and placedinto an array or any other suitable arrangement.

FIG. 5B schematically shows that the chips 810 are attached to a supportwafer 820. For example, the chips 810 maybe attached with an adhesive.

FIG. 5C schematically shows that the chips 810 are mounted to thesubstrate 830 while still attached to the support wafer 820. Thesubstrate 830 can be the X-ray absorption layer 110 depicted in FIG. 4C.

FIG. 5D schematically shows an alternative, where the chips 810 attachedto a single support wafer 820 may be mounted to multiple substrates 830.

FIG. 5E schematically shows an alternative, where the chips 810 aremounted to multiple substrates 830 but the boundaries among the chips810 and the boundaries among the substrate 830 may not coincide. Namely,the electrical contacts 125 on a given chip 810 may be connected todifferent substrates 830 and the electrical contacts 119B on a givensubstrate 830 may be connected to different chips 810. The chips 810 andthe substrates 830 may both include transmission lines and contactsconfigured to connect the transmission lines in them. For clarity, somecomponents of the chips 810 and the substrates 830 are omitted in FIG.5E.

FIG. 5F schematically shows that the support wafer 820 is removed. Forexample, the support wafer 820 may be ground away, etched away, orseparated from the chips 810.

FIG. 6A-FIG. 6E schematically show a process of mounting a plurality ofchips onto a substrate, according to an embodiment. This process may beused in mounting the chips depicted in FIG. 4C.

FIG. 6A schematically shows that chips 910 (e.g., the chip including theelectronic layer 120 as shown in FIG. 4C) may be obtained and placedinto an array or any other suitable arrangement on a support wafer 920.

FIG. 6B schematically shows that the chips 910 are encapsulated in amatrix 925. The matrix 925 is supported on the support wafer 920. Thematrix 925 may be a polymer, glass or other suitable material. Thematrix 925 may fill gaps between the chips 910.

FIG. 6C schematically shows that the support wafer 920 is removed. Forexample, the support wafer 920 may be ground away, etched away, orseparated from the chips 910. The matrix 925 supports the chips 910after removal of the support wafer 920. The surfaces of the chips 910contacting the support wafer 920 may be exposed by removal of thesupport wafer 920.

FIG. 6D schematically shows the encapsulated chips 910 are aligned tothe substrate 930. The chips 910 may be then aligned with the substrate930 (e.g. the X-ray absorption layer 110 depicted in FIG. 4C), orstructures thereon (e.g., electrical contacts)

FIG. 6E schematically shows that the encapsulated chips 910 are attachedto the substrate 930.

FIG. 7A schematically shows that the chips including the electroniclayer 120 may have vias 126 extending to a surface opposite to the X-rayabsorption layer 110, to which the chips are mounted.

FIG. 7B schematically shows that the vias 126 may be aligned to contactspads 410 on interposer substrate 400 (e.g., a silicon wafer).

FIG. 7C schematically shows that the electronic layers 120 of the chipsare bonded to the interposer substrate 400. After the bonding, the vias126 are electrically connected contacts pads 410. The interposersubstrate 400 may have transmission lines buried in the interposersubstrate 400 or on the surface on the interposer substrate 400. Thetransmission lines are electrically connected to the contact pads 410and are configured to route signals on the contact pads 410 to bondingpads 430 on the edge of the interposer substrate 400. The interposersubstrate 400 is mounted to a printed circuit board 500. Alternatively,the interposer substrate 400 may be positioned side by side with aprinted circuit board 500. More than one interposer substrate may bemounted to the same printed circuit board. The electrical contactbetween the interposer substrate 400 and the printed circuit board 500may be made with wire bonding.

FIG. 8A schematically shows that the chips including the electroniclayer 120 are bonded to the X-ray absorption layer 110.

FIG. 8B schematically shows that the X-ray absorption layer 110 isoptionally mounted to a printed circuit board 600. Alternatively, theX-ray absorption layer 110 may be optionally positioned side by sidewith a printed circuit board 600. Electrical contact between the X-rayabsorption layer 110 and the printed circuit board 600 may be made withwire bonding.

FIG. 8C schematically shows that the first surface 124 of the electroniclayer 120 has a set of electrical contacts 129 in addition to theelectrical contacts 125. The electrical contacts 129 may function as I/Ointerface to the electronic system 121. For example the electricalcontacts 129 may be configured to read output from the electronic system121, controlling the electronic system 121, or provide power orreference voltages to the electronic system 121.

As schematically shown in FIG. 8D and FIG. 8E, the X-ray absorptionlayer 110 may have electrical contacts 119C configured to connect withthe electrical contacts 129 after the chips are mounted to the X-rayabsorption layer 110. The X-ray absorption layer 110 may havetransmission lines 119E configured to route signals at the electricalcontacts 119C to bonding pads 119D near the edge of the X-ray absorptionlayer 110. The bonding pads 119D may be used to make electricalconnections to a PCB or to another integrated circuit. The electricalcontacts 119C may be at locations where some of the electrical contact11913 would otherwise be, as shown in FIG. 8D. The electrical contacts119C may be located in area between the electrical contact 119B, asshown in FIG. 8E.

FIGS. 8F-81 schematically show routing of signal in the X-ray absorptionlayer 110 and the electronic layer 120. The features of the X-rayabsorption layer 110 such as the electrical contacts 119A and 119B andthe discrete regions 114 are usually on the scale of micrometers. Thefeatures of the X-ray absorption layer 110 may be fabricated by doingwhole-wafer lithography. The features of the electronic layer 120 areusually much smaller and may not be fabricated by doing whole-waferlithography. Instead, the features of the electronic layer 120 may befabricated by doing die-by-die lithography. Therefore, makinglong-distance transmission lines (e.g., across a whole 8″ wafer) on theX-ray absorption layer 110 is much easier than making transmission linesacross boundaries between the dies.

As shown in FIG. 8F, where multiples chips containing the X-rayabsorption layer 110 and multiple chips containing the electronic layer120 are attached, the chips containing the X-ray absorption layer 110may include electrical contacts 891, bonding pads 892, transmissionlines 893 connecting the electrical contacts 891 to the bonding pads892; the chips containing the electronic layer 120 may includeelectrical contacts 894 and transmission lines 895 connecting among theelectrical contacts 894. The transmissions lines 895 do not crossboundaries of dies. The electrical contacts 891 and the electricalcontacts 894 may be aligned and connected such that signals are bridgedacross gaps between the chips containing the electronic layer 120through the transmission lines 893 and across gaps between the chipscontaining the X-ray absorption layer 110 through the transmission lines895.

As shown in FIG. 8G, where multiples chips containing the X-rayabsorption layer 110 and a wafer containing the electronic layer 120 areattached, the chips containing the X-ray absorption layer 110 mayinclude electrical contacts 891, bonding pads 892, transmission lines893 connecting the electrical contacts 891 to the bonding pads 892; thewafer containing the electronic layer 120 may include electricalcontacts 894 and transmission lines 895 connecting among the electricalcontacts 894. The transmissions lines 895 do not cross boundaries ofdies. The electrical contacts 891 and the electrical contacts 894 may bealigned and connected such that signals are bridged across gaps betweenthe chips containing the X-ray absorption layer 110 through thetransmission lines 895.

As shown in FIG. 8H, where a wafer containing the X-ray absorption layer110 and multiple chips containing the electronic layer 120 are attached,the wafer containing the X-ray absorption layer 110 may includeelectrical contacts 891, bonding pads 892, transmission lines 893connecting the electrical contacts 891 to the bonding pads 892; thechips containing the electronic layer 120 may include electricalcontacts 894 but do not have to include transmission lines connectingamong the electrical contacts 894. The electrical contacts 891 and theelectrical contacts 894 may be aligned and connected such that signalsare bridged across gaps between the chips containing the electroniclayer 120 through the transmission lines 893.

As shown in FIG. 8I, where a wafer containing the X-ray absorption layer110 and a wafer containing the electronic layer 120 are attached, thewafer containing the X-ray absorption layer 110 may include electricalcontacts 891, bonding pads 892, transmission lines 893 connecting theelectrical contacts 891 to the bonding pads 892; the wafer containingthe electronic layer 120 may include electrical contacts 894 but do nothave to include transmission lines connecting among the electricalcontacts 894. The electrical contacts 891 and the electrical contacts894 may be aligned and connected such that signals from the electroniclayer 120 are routed through the transmission lines 893.

FIG. 9 schematically shows a system comprising the semiconductor X-raydetector 100 described herein. The system may be used for medicalimaging such as chest X-ray radiography, abdominal X-ray radiography,etc. The system comprises an X-ray source 1201. X-ray emitted from theX-ray source 1201 penetrates an object 1202 (e.g., a human body partsuch as chest, limb, abdomen), is attenuated by different degrees by theinternal structures of the object 1202 (e.g., bones, muscle, fat andorgans, etc.), and is projected to the semiconductor X-ray detector 100.The semiconductor X-ray detector 100 forms an image by detecting theintensity distribution of the X-ray.

FIG. 10 schematically shows a system comprising the semiconductor X-raydetector 100 described herein. The system may be used for medicalimaging such as dental X-ray radiography. The system comprises an X-raysource 1301. X-ray emitted from the X-ray source 1301 penetrates anobject 1302 that is part of a mammal (e.g., human) mouth. The object1302 may include a maxilla bone, a palate bone, a tooth, the mandible,or the tongue. The X-ray is attenuated by different degrees by thedifferent structures of the object 1302 and is projected to thesemiconductor X-ray detector 100. The semiconductor X-ray detector 100forms an image by detecting the intensity distribution of the X-ray.Teeth absorb X-ray more than dental caries, infections, periodontalligament. The dosage of X-ray radiation received by a dental patient istypically small (around 0.150 mSv for a full mouth series).

FIG. 11 schematically shows a cargo scanning or non-intrusive inspection(NII) system comprising the semiconductor X-ray detector 100 describedherein. The system may be used for inspecting and identifying goods intransportation systems such as shipping containers, vehicles, ships,luggage, etc. The system comprises an X-ray source 1401. X-ray emittedfrom the X-ray source 1401 may backscatter from an object 1402 (e.g.,shipping containers, vehicles, ships, etc.) and be projected to thesemiconductor X-ray detector 100. Different internal structures of theobject 1402 may backscatter X-ray differently. The semiconductor X-raydetector 100 forms an image by detecting the intensity distribution ofthe backscattered X-ray and/or energies of the backscattered X-rayphotons.

FIG. 12 schematically shows another cargo scanning or non-intrusiveinspection (NII) system comprising the semiconductor X-ray detector 100described herein. The system may be used for luggage screening at publictransportation stations and airports. The system comprises an X-raysource 1501. X-ray emitted from the X-ray source 1501 may penetrate apiece of luggage 1502, be differently attenuated by the contents of theluggage, and projected to the semiconductor X-ray detector 100. Thesemiconductor X-ray detector 100 forms an image by detecting theintensity distribution of the transmitted X-ray. The system may revealcontents of luggage and identify items forbidden on publictransportation, such as firearms, narcotics, edged weapons, flammables.

FIG. 13 schematically shows a full-body scanner system comprising thesemiconductor X-ray detector 100 described herein. The full-body scannersystem may detect objects on a person's body for security screeningpurposes, without physically removing clothes or making physicalcontact. The full-body scanner system may be able to detect non-metalobjects. The full-body scanner system comprises an X-ray source 1601.X-ray emitted from the X-ray source 1601 may backscatter from a human1602 being screened and objects thereon, and be projected to thesemiconductor X-ray detector 100. The objects and the human body maybackscatter X-ray differently. The semiconductor X-ray detector 100forms an image by detecting the intensity distribution of thebackscattered X-ray. The semiconductor X-ray detector 100 and the X-raysource 1601 may be configured to scan the human in a linear orrotational direction.

FIG. 14 schematically shows an X-ray computed tomography (X-ray CT)system. The X-ray CT system uses computer-processed X-rays to producetomographic images (virtual “slices”) of specific areas of a scannedobject. The tomographic images may be used for diagnostic andtherapeutic purposes in various medical disciplines, or for flawdetection, failure analysis, metrology, assembly analysis and reverseengineering. The X-ray CT system comprises the semiconductor X-raydetector 100 described herein and an X-ray source 1701. Thesemiconductor X-ray detector 100 and the X-ray source 1701 may beconfigured to rotate synchronously along one or more circular or spiralpaths.

FIG. 15 schematically shows an electron microscope. The electronmicroscope comprises an electron source 1801 (also called an electrongun) that is configured to emit electrons. The electron source 1801 mayhave various emission mechanisms such as thermionic, photocathode, coldemission, or plasmas source. The emitted electrons pass through anelectronic optical system 1803, which gray be configured to shape,accelerate, or focus the electrons. The electrons then reach a sample1802 and an image detector may form an image therefrom. The electronmicroscope may comprise the semiconductor X-ray detector 100 describedherein, for performing energy-dispersive X-ray spectroscopy (EDS). EDSis an analytical technique used for the elemental analysis or chemicalcharacterization of a sample. When the electrons incident on a sample,they cause emission of characteristic X-rays from the sample. Theincident electrons may excite an electron in an inner shell of an atomin the sample, ejecting it from the shell while creating an electronhole where the electron was. An electron from an outer, higher-energyshell then fills the hole, and the difference in energy between thehigher-energy shell and the lower energy shell may be released in theform of an X-ray. The number and energy of the X-rays emitted from thesample can be measured by the semiconductor X-ray detector 100.

The semiconductor X-ray detector 100 described here may have otherapplications such as in an X-ray telescope, X-ray mammography,industrial X-ray defect detection, X-ray microscopy or microradiography,X-ray casting inspection, X-ray non-destructive testing, X-ray weldinspection, X-ray digital subtraction angiography, etc. It may besuitable to use this semiconductor X-ray detector 100 in place of aphotographic plate, a photographic film, a PSP plate, an X-ray imageintensifier, a scintillator, or another semiconductor X-ray detector.

FIG. 16A and FIG. 16B each show a component diagram of the electronicsystem 121, according to an embodiment. The electronic system 121 mayinclude a first voltage comparator 301, a second voltage comparator 302,a counter 320, a switch 305, a voltmeter 306 and a controller 310.

The first voltage comparator 301 is configured to compare the voltage ofan electrode of a diode 300 to a first threshold. The diode may be adiode formed by the first doped region 111, one of the discrete regions114 of the second doped region 113, and the optional intrinsic region112. Alternatively, the first voltage comparator 301 is configured tocompare the voltage of an electrical contact (e.g., a discrete portionof electrical contact 119B) to a first threshold. The first voltagecomparator 301 may be configured to monitor the voltage directly, orcalculate the voltage by integrating an electric current flowing throughthe diode or electrical contact over a period of time. The first voltagecomparator 301 may be controllably activated or deactivated by thecontroller 310. The first voltage comparator 301 may be a continuouscomparator. Namely, the first voltage comparator 301 may be configuredto be activated continuously, and monitor the voltage continuously. Thefirst voltage comparator 301 configured as a continuous comparatorreduces the chance that the system 121 misses signals generated by anincident X-ray photon. The first voltage comparator 301 configured as acontinuous comparator is especially suitable when the incident X-rayintensity is relatively high. The first voltage comparator 301 may be aclocked comparator, which has the benefit of lower power consumption.The first voltage comparator 301 configured as a clocked comparator maycause the system 121 to miss signals generated by some incident X-rayphotons. When the incident X-ray intensity is low, the chance of missingan incident X-ray photon is low because the time interval between twosuccessive photons is relatively long. Therefore, the first voltagecomparator 301 configured as a clocked comparator is especially suitablewhen the incident X-ray intensity is relatively low. The first thresholdmay be 5-10%, 10%-20%, 20-30%, 30-40% or 40-50% of the maximum voltageone incident X-ray photon may generate in the diode or the resistor. Themaximum voltage may depend on the energy of the incident X-ray photon(i.e., the wavelength of the incident X-ray), the material of the X-rayabsorption layer 110, and other factors. For example, the firstthreshold may be 50 mV, 100 mV, 150 mV, or 200 mV.

The second voltage comparator 302 is configured to compare the voltageto a second threshold. The second voltage comparator 302 may beconfigured to monitor the voltage directly, or calculate the voltage byintegrating an electric current flowing through the diode or theelectrical contact over a period of time. The second voltage comparator302 may be a continuous comparator. The second voltage comparator 302may be controllably activate or deactivated by the controller 310. Whenthe second voltage comparator 302 is deactivated, the power consumptionof the second voltage comparator 302 may be less than 1%, less than 5%,less than 10% or less than 20% of the power consumption when the secondvoltage comparator 302 is activated. The absolute value of the secondthreshold is greater than the absolute value of the first threshold. Asused herein, the term “absolute value” or “modulus” |x| of a real numberx is the non-negative value of x without regard to its sign. Namely,

${x} = \left\{ {\begin{matrix}{x,{{{if}\mspace{14mu} x} \geq 0}} \\{{- x},{{{if}\mspace{14mu} x} \leq 0}}\end{matrix}.} \right.$

The second threshold may be 200%-300% of the first threshold. The secondthreshold may be at least 50% of the maximum voltage one incident X-rayphoton may generate in the diode or resistor. For example, the secondthreshold may be 100 mV, 150 mV, 200 mV, 250 mV or 300 mV. The secondvoltage comparator 302 and the first voltage comparator 310 may be thesame component. Namely, the system 121 may have one voltage comparatorthat can compare a voltage with two different thresholds at differenttimes.

The first voltage comparator 301 or the second voltage comparator 302may include one or more op-amps or any other suitable circuitry. Thefirst voltage comparator 301 or the second voltage comparator 302 mayhave a high speed to allow the system 121 to operate under a high fluxof incident X-ray. However, having a high speed is often at the cost ofpower consumption.

The counter 320 is configured to register a number of X-ray photonsreaching the diode or resistor. The counter 320 may be a softwarecomponent (e.g., a number stored in a computer memory) or a hardwarecomponent (e.g., a 4017 IC and a 7490 IC).

The controller 310 may be a hardware component such as a microcontrollerand a microprocessor. The controller 310 is configured to start a timedelay from a time at which the first voltage comparator 301 determinesthat the absolute value of the voltage equals or exceeds the absolutevalue of the first threshold (e.g., the absolute value of the voltageincreases from below the absolute value of the first threshold to avalue equal to or above the absolute value of the first threshold). Theabsolute value is used here because the voltage may be negative orpositive, depending on whether the voltage of the cathode or the anodeof the diode or which electrical contact is used. The controller 310 maybe configured to keep deactivated the second voltage comparator 302, thecounter 320 and any other circuits the operation of the first voltagecomparator 301 does not require, before the time at which the firstvoltage comparator 301 determines that the absolute value of the voltageequals or exceeds the absolute value of the first threshold. The timedelay may expire before or after the voltage becomes stable, i.e., therate of change of the voltage is substantially zero. The phase “the rateof change of the voltage is substantially zero” means that temporalchange of the voltage is less than 0.1%/ns. The phase “the rate ofchange of the voltage is substantially non-zero” means that temporalchange of the voltage is at least 0.1%/ns.

The controller 310 may be configured to activate the second voltagecomparator during (including the beginning and the expiration) the timedelay. In an embodiment, the controller 310 is configured to activatethe second voltage comparator at the beginning of the time delay. Theterm “activate” means causing the component to enter an operationalstate (e.g., by sending a signal such as a voltage pulse or a logiclevel, by providing power, etc.). The term “deactivate” means causingthe component to enter a non-operational state (e.g., by sending asignal such as a voltage pulse or a logic level, by cut off power,etc.). The operational state may have higher power consumption (e.g., 10times higher, 100 times higher, 1000 times higher) than thenon-operational state. The controller 310 itself may be deactivateduntil the output of the first voltage comparator 301 activates thecontroller 310 when the absolute value of the voltage equals or exceedsthe absolute value of the first threshold.

The controller 310 may be configured to cause the number registered bythe counter 320 to increase by one, if, during the time delay, thesecond voltage comparator 302 determines that the absolute value of thevoltage equals or exceeds the absolute value of the second threshold.

The controller 310 gray be configured to cause the voltmeter 306 tomeasure the voltage upon expiration of the time delay. The controller310 may be configured to connect the electrode to an electrical ground,so as to reset the voltage and discharge any charge carriers accumulatedon the electrode. In an embodiment, the electrode is connected to anelectrical ground after the expiration of the time delay. In anembodiment, the electrode is connected to an electrical ground for afinite reset time period. The controller 310 may connect the electrodeto the electrical ground by controlling the switch 305. The switch maybe a transistor such as a field-effect transistor (FET).

In an embodiment, the system 121 has no analog filter network (e.g., aRC network). In an embodiment, the system 121 has no analog circuitry.

The voltmeter 306 may feed the voltage it measures to the controller 310as an analog or digital signal.

The system 121 may include a capacitor module 309 electrically connectedto the electrode of the diode 300 or which electrical contact, whereinthe capacitor module is configured to collect charge carriers from theelectrode. The capacitor module can include a capacitor in the feedbackpath of an amplifier. The amplifier configured as such is called acapacitive transimpedance amplifier (CTIA). CTIA has high dynamic rangeby keeping the amplifier from saturating and improves thesignal-to-noise ratio by limiting the bandwidth in the signal path.Charge carriers from the electrode accumulate on the capacitor over aperiod of time (“integration period”) (e.g., as shown FIG. 17, betweent₀ to t₁, or t₁-t₂). After the integration period has expired, thecapacitor voltage is sampled and then reset by a reset switch. Thecapacitor module can include a capacitor directly connected to theelectrode.

FIG. 17 schematically shows a temporal change of the electric currentflowing through the electrode (upper curve) caused by charge carriersgenerated by an X-ray photon incident on the diode or the resistor, anda corresponding temporal change of the voltage of the electrode (lowercurve). The voltage may be an integral of the electric current withrespect to time. At time t₀, the X-ray photon hits the diode or theresistor, charge carriers start being generated in the diode or theresistor, electric current starts to flow through the electrode of thediode or the resistor, and the absolute value of the voltage of theelectrode or electrical contact starts to increase. At time t₁, thefirst voltage comparator 301 determines that the absolute value of thevoltage equals or exceeds the absolute value of the first threshold V1,and the controller 310 starts the time delay TD1 and the controller 310may deactivate the first voltage comparator 301 at the beginning of TD1.If the controller 310 is deactivated before t₁, the controller 310 isactivated at t₁. During TD1, the controller 310 activates the secondvoltage comparator 302. The term “during” a time delay as used heremeans the beginning and the expiration (i.e., the end) and any time inbetween. For example, the controller 310 may activate the second voltagecomparator 302 at the expiration of TD1. If during TD1, the secondvoltage comparator 302 determines that the absolute value of the voltageequals or exceeds the absolute value of the second threshold at time t₂,the controller 310 causes the number registered by the counter 320 toincrease by one. At time t_(e), all charge carriers generated by theX-ray photon drift out of the X-ray absorption layer 110. At time t_(s),the time delay TD1 expires. In the example of FIG. 17, time t_(s) isafter time t_(e); namely TD1 expires after all charge carriers generatedby the X-ray photon drift out of the X-ray absorption layer 110. Therate of change of the voltage is thus substantially zero at t_(s). Thecontroller 310 may be configured to deactivate the second voltagecomparator 302 at expiration of TD1 or at t₂, or any time in between.

The controller 310 may be configured to cause the voltmeter 306 tomeasure the voltage upon expiration of the time delay TD1. In anembodiment, the controller 310 causes the voltmeter 306 to measure thevoltage after the rate of change of the voltage becomes substantiallyzero after the expiration of the time delay TD1. The voltage at thismoment is proportional to the amount of charge carriers generated by anX-ray photon, which relates to the energy of the X-ray photon. Thecontroller 310 may be configured to determine the energy of the X-rayphoton based on voltage the voltmeter 306 measures. One way to determinethe energy is by binning the voltage. The counter 320 may have asub-counter for each bin. When the controller 310 determines that theenergy of the X-ray photon falls in a bin, the controller 310 may causethe number registered in the sub-counter for that bin to increase byone. Therefore, the system 121 may be able to detect an X-ray image andmay be able to resolve X-ray photon energies of each X-ray photon.

After TD1 expires, the controller 310 connects the electrode to anelectric ground for a reset period RST to allow charge carriersaccumulated on the electrode to flow to the ground and reset thevoltage. After RST, the system 121 is ready to detect another incidentX-ray photon. Implicitly, the rate of incident X-ray photons the system121 can handle in the example of FIG. 17 is limited by 1/(TD1+RST). Ifthe first voltage comparator 301 has been deactivated, the controller310 can activate it at any time before RST expires. If the controller310 has been deactivated, it may be activated before RST expires.

FIG. 18 schematically shows a temporal change of the electric currentflowing through the electrode (upper curve) caused by noise (e.g., darkcurrent, background radiation, scattered X-rays, fluorescent X-rays,shared charges from adjacent pixels), and a corresponding temporalchange of the voltage of the electrode (lower curve), in the system 121operating in the way shown in FIG. 17. At time t₀, the noise begins. Ifthe noise is not large enough to cause the absolute value of the voltageto exceed the absolute value of V1, the controller 310 does not activatethe second voltage comparator 302. If the noise is large enough to causethe absolute value of the voltage to exceed the absolute value of V1 attime t₁ as determined by the first voltage comparator 301, thecontroller 310 starts the time delay TD1 and the controller 310 maydeactivate the first voltage comparator 301 at the beginning of TD1.During TD1 (e.g., at expiration of TD1), the controller 310 activatesthe second voltage comparator 302. The noise is very unlikely largeenough to cause the absolute value of the voltage to exceed the absolutevalue of V2 during TD1. Therefore, the controller 310 does not cause thenumber registered by the counter 320 to increase. At time t_(e), thenoise ends. At time t_(s), the time delay TD1 expires. The controller310 may be configured to deactivate the second voltage comparator 302 atexpiration of TD1. The controller 310 may be configured not to cause thevoltmeter 306 to measure the voltage if the absolute value of thevoltage does not exceed the absolute value of V2 during TD1. After TD1expires, the controller 310 connects the electrode to an electric groundfor a reset period RST to allow charge carriers accumulated on theelectrode as a result of the noise to flow to the ground and reset thevoltage. Therefore, the system 121 may be very effective in noiserejection.

FIG. 19 schematically shows a temporal change of the electric currentflowing through the electrode (upper curve) caused by charge carriersgenerated by an X-ray photon incident on the diode or the resistor, anda corresponding temporal change of the voltage of the electrode (lowercurve), when the system 121 operates to detect incident X-ray photons ata rate higher than 1/(TD1+RST). The voltage may be an integral of theelectric current with respect to time. At time t₀, the X-ray photon hitsthe diode or the resistor, charge carriers start being generated in thediode or the resistor, electric current starts to flow through theelectrode of the diode or the electrical contact of resistor, and theabsolute value of the voltage of the electrode or the electrical contactstarts to increase. At time t₁, the first voltage comparator 301determines that the absolute value of the voltage equals or exceeds theabsolute value of the first threshold V1, and the controller 310 startsa time delay TD2 shorter than TD1, and the controller 310 may deactivatethe first voltage comparator 301 at the beginning of TD2. If thecontroller 310 is deactivated before t₁, the controller 310 is activatedat t₁. During TD2 (e.g., at expiration of TD2), the controller 310activates the second voltage comparator 302. If during TD2, the secondvoltage comparator 302 determines that the absolute value of the voltageequals or exceeds the absolute value of the second threshold at time t₂,the controller 310 causes the number registered by the counter 320 toincrease by one. At time t_(e), all charge carriers generated by theX-ray photon drift out of the X-ray absorption layer 110. At time t_(h),the time delay TD2 expires. In the example of FIG. 19, time t_(h) isbefore time t_(e); namely TD2 expires before all charge carriersgenerated by the X-ray photon drift out of the X-ray absorption layer110. The rate of change of the voltage is thus substantially non-zero att_(h). The controller 310 may be configured to deactivate the secondvoltage comparator 302 at expiration of TD2 or at t₂, or any time inbetween.

The controller 310 may be configured to extrapolate the voltage at t_(e)from the voltage as a function of time during TD2 and use theextrapolated voltage to determine the energy of the X-ray photon.

After TD2 expires, the controller 310 connects the electrode to anelectric ground for a reset period RST to allow charge carriersaccumulated on the electrode to flow to the ground and reset thevoltage. In an embodiment, RST expires before t_(e). The rate of changeof the voltage after RST may be substantially non-zero because allcharge carriers generated by the X-ray photon have not drifted out ofthe X-ray absorption layer 110 upon expiration of RST before t_(e). Therate of change of the voltage becomes substantially zero after t_(e) andthe voltage stabilized to a residue voltage VR after t_(e). In anembodiment, RST expires at or after t_(e), and the rate of change of thevoltage after RST may be substantially zero because all charge carriersgenerated by the X-ray photon drift out of the X-ray absorption layer110 at t_(e). After RST, the system 121 is ready to detect anotherincident X-ray photon. If the first voltage comparator 301 has beendeactivated, the controller 310 can activate it at any time before RSTexpires. If the controller 310 has been deactivated, it may be activatedbefore RST expires.

FIG. 20 schematically shows a temporal change of the electric currentflowing through the electrode (upper curve) caused by noise (e.g., darkcurrent, background radiation, scattered X-rays, fluorescent X-rays,shared charges from adjacent pixels), and a corresponding temporalchange of the voltage of the electrode (lower curve), in the system 121operating in the way shown in FIG. 19. At time t₀, the noise begins. Ifthe noise is not large enough to cause the absolute value of the voltageto exceed the absolute value of V1, the controller 310 does not activatethe second voltage comparator 302. If the noise is large enough to causethe absolute value of the voltage to exceed the absolute value of V1 attime t₁ as determined by the first voltage comparator 301, thecontroller 310 starts the time delay TD2 and the controller 310 maydeactivate the first voltage comparator 301 at the beginning of TD2.During TD2 (e.g., at expiration of TD2), the controller 310 activatesthe second voltage comparator 302. The noise is very unlikely largeenough to cause the absolute value of the voltage to exceed the absolutevalue of V2 during TD2. Therefore, the controller 310 does not cause thenumber registered by the counter 320 to increase. At time t_(e), thenoise ends. At time t_(h), the time delay TD2 expires. The controller310 may be configured to deactivate the second voltage comparator 302 atexpiration of TD2. After TD2 expires, the controller 310 connects theelectrode to an electric ground for a reset period RST to allow chargecarriers accumulated on the electrode as a result of the noise to flowto the ground and reset the voltage. Therefore, the system 121 may bevery effective in noise rejection.

FIG. 21 schematically shows a temporal change of the electric currentflowing through the electrode (upper curve) caused by charge carriersgenerated by a series of X-ray photons incident on the diode or theresistor, and a corresponding temporal change of the voltage of theelectrode (lower curve), in the system 121 operating in the way shown inFIG. 19 with RST expires before t_(e). The voltage curve caused bycharge carriers generated by each incident X-ray photon is offset by theresidue voltage before that photon. The absolute value of the residuevoltage successively increases with each incident photon. When theabsolute value of the residue voltage exceeds V1 (see the dottedrectangle in FIG. 21), the controller starts the time delay TD2 and thecontroller 310 may deactivate the first voltage comparator 301 at thebeginning of TD2. If no other X-ray photon incidence on the diode or theresistor during TD2, the controller connects the electrode to theelectrical ground during the reset time period RST at the end of TD2,thereby resetting the residue voltage. The residue voltage thus does notcause an increase of the number registered by the counter 320.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

What is claimed is:
 1. A method for making an apparatus suitable fordetecting X-ray, the method comprising: bonding a plurality of chips toa substrate; wherein the substrate comprises an X-ray absorption layercomprising a first plurality of electrical contacts; wherein each of theplurality of chips comprises an electronic layer comprising a secondplurality of electrical contacts and an electronic system configured toprocess or interpret signals generated by X-ray photons incident on theX-ray absorption layer; aligning the first plurality of electricalcontacts to the second plurality of electrical contacts; mounting thechips to the substrate such that the first plurality of electricalcontacts are electrically connected to the second plurality ofelectrical contacts; wherein the second plurality of electrical contactsare configured to feed the signals to the electronic system.
 2. Themethod of claim 1, further comprising attaching the plurality of chipsto a support wafer.
 3. The method of claim 2, wherein the plurality ofchips are attached to the support wafer with an adhesive.
 4. The methodof claim 2, wherein the plurality of chips are attached to the supportwafer after the plurality of chips are mounted to the substrate.
 5. Themethod of claim 2, wherein the plurality of chips are mounted to asecond substrate.
 6. The method of claim 2, further comprising removingthe support wafer.
 7. The method of claim 6, wherein removing thesupport wafer comprises grinding or etching the support wafer.
 8. Themethod of claim 1, further comprising encapsulating the plurality ofchips in a matrix.
 9. The method of claim 8, wherein the matrixcomprises a polymer or glass.
 10. The method of claim 8, wherein thematrix fills gaps between the chips.
 11. The method of claim 8, furthercomprising exposing a surface of each of the chips.
 12. The method ofclaim 11, wherein mounting the chips to the substrate comprises mountingthe chips encapsulated in the matrix.
 13. The method of claim 1, whereinthe electronic layer comprises vias extending to a surface opposite tothe X-ray absorption layer.
 14. The method of claim 13, furthercomprising aligning the vias to contact pads on an interposer substrate,and bonding the chips to the interposer substrate such that the vias areelectrically connected to the contact pads.
 15. The method of claim 14,wherein the interposer substrate comprises transmission lineselectrically connected to the contact pads and configured to route asignal on the contact pads to bonding pads on an edge of the interposersubstrate.
 16. The method of claim 15, further comprising mounting theinterposer substrate to a printed circuit board or positioning theinterposer substrate side-by-side with a printed circuit board.
 17. Themethod of claim 1, wherein the electronic layer comprises a thirdplurality of electrical contacts configured to read output from theelectronic system or to provide power or a reference voltage to theelectronic system.
 18. The method of claim 17, wherein the X-rayabsorption layer comprises a fourth plurality of electrical contactsconfigured to connect with the third electrical contacts when the chipsare mounted to the substrate.
 19. The method of claim 18, wherein theX-ray absorption layer further comprises transmission lines configuredto route a signal at the fourth plurality of electrical contacts tobonding pads on the X-ray absorption layer.